This study quantifies the histologic and cell kinetic parameters of the irradiated swine skin cell renewal system. These data have not been obtained since the histologic changes were first decribed in 1913. The first year documents swine skin response produced by several time-exposure schedules using moist reactions as the parameter. The next year, histologic and cell kinetic parameters are quantified after delivery of 200 rads/day, 5x week for 5-8 weeks (schedule most used clinically). After defining cell parameters, the population response can be correlated with the evolution, time course and dose dependence of the moist reaction. The dose required to produce a given effect is increased when delivered in multiple increments. Mechanisms of dose recovery have not been designated, though molecular repair and repopulation are proposed. The role of repopulation is to be assessed by measuring mitotic and cell density changes occurring at mid-week and over week-ends during irradiation. By defining these parameters, an in vivo test system becomes available equivalent to in vitro tissue culture monolayers. Consequently, the skin model is ideal to evaluate new time-exposure schedules. For the first time, these schedules can be determined on a rational rather than empirical basis.